306 Notices respecting New Books. 



field is therefore used in maintaining the vibrations of these pairs 

 of ions, which are damped by electrical and mechanical forces. 

 By writing down the equations of energy and applying the 

 principle of least action, results are obtained which represent 

 the facts of dispersion and absorption with a fair degree of 

 accuracy. The motion of the charged ions in a magnetic field 

 gives rise to movements of the aether which set up electric and 

 ponderomotive forces ; the effects of these are compounded with 

 the electric and magnetic vibrations in the light waves, giving rise 

 to two circularly-polarized waves which travel with different 

 velocities. The theory has recently received some confirmation 

 from Zeeman's observations on the existence and polarization of 

 broadened spectral lines from a source of light in a magnetic 

 field. 



The volume is well printed, and appears to be remarkably free 

 from the typographical errors which often occur in works of a 

 mathematical character. The series, when complete, will rank as 

 one of the most important treatises on modern physics. J. L. H. 



Magnetic Fields of Force. By H. Ebert, Professor of Physics in 



the University of Kiel. Translated by 0. Y. Burton, D.Sc. 



Parti. London: Longmans, 1897. 

 So much has been written lately concerning scientific education 

 in Germany, to the disparagement of this country, that it is 

 encouraging to meet with a modern text-book for German students 

 in which the principles adopted are essentially of British origin. 

 By the labours of Faraday and Maxwell we were early taught the 

 usefulness of the conception of lines of force, in explaining both 

 electric and magnetic phenomena ; it has, however, required the 

 work of Hertz to finally eradicate from Germany and from science 

 the theories of distance-action. Professor Ebert presents in this 

 volume the principal facts pertaining to magnetism and electro- 

 magnetism, and shows how they may be explained qualitatively 

 and quantitatively by assuming that the maguetic field contains 

 energy, and that force exists at every point in it. Venturing a 

 step further, he quotes Lord Kelvin's deduction from the phe- 

 nomenon of magnetic rotation of the plane of polarization, that 

 lines of force are kinematically comparable with axes of rotation, 

 and advances the theory that magnetism is due to a rotational 

 motion. This leads to an interesting chapter on vortex motion 

 and on cyclic motions in general ; the latter subject, which was 

 first treated by Helmholtz and Boltzmann and has received further 

 development in Hertz's ' Principles of Mechanics,' serves as an 

 introduction to the second part of the author's work. 



We regret to note that the dimensions of magnetic quantities 

 are specified on the assumption that permeability is a mere ratio. 

 This is all the more remarkable in a treatise which traces magnetic 

 phenomena to a motion of the aether, because the nature of such 

 motion determines to some extent the absolute dimensions of the 

 permeability of the medium. 



Dr. Burton has performed the duties of translator with great 



